Ferroelectric materials are dielectrics which can be used in memory circuits. One physical property useful in this regard relates to a hysteresis curve concerning the applied field and the polarization of the ferroelectric material. As described in U.S. Pat. No. 3,728,694, for example, a capacitor can be formed having a ferroelectric material between the electrodes to serve as the dielectric. Because of the hysteresis curve, when an applied voltage is removed, the polarization of the ferroelectric material will be in one of two distinct states. These states can be determined in various fashions, typically by applying a pulse and determining the amount of current passing through the capacitor.
Data retention refers to maintaining the physical phenomenon or property by which binary data is stored, such as ferroelectric polarization, without applied voltage. While ferroelectric materials are useful in non-volatile memory, it is desirable and important that the non-volatile memory be able to maintain the data stored therein for long periods of time, such as years. That is to say, the objective with a ferroelectric, non-volatile memory is to keep the dipoles oriented properly, without changing direction. This has been a problem in using ferroelectric materials. Consequently, an important object of this invention is to provide a way to preserve the polarization of ferroelectric materials after it has been set, without providing further power to the material, i.e. by an applied voltage.
Another problem with ferroelectric materials relates to dipole relaxation. If the material has been in one of its two different polarization states for a relatively long time, one nevertheless wants to be able to change the polarization with a relatively short voltage pulse. For example, if a capacitor using ferroelectric material as dielectric has been in one state for a relatively long time, such as one year, and then a 100 nanosecond pulse is applied to the capacitor plates in such a direction as to change the orientation of the dipoles, then after the pulse is terminated, the dipoles may relax back to the original orientation. Problems of this nature are greater when the ferroelectric material is in a relatively thin film of below about one micron. Therefore, a further object of this invention is to provide a way to overcome the relaxation problem of ferroelectric materials.